1. Field of the Invention
The present invention relates to superparamagnetic core shell nanoparticles having an iron oxide core and a silica shell which have high magnetic saturation and a magnetic core produced with these high magnetic saturation nanoparticles. The core of the present invention is suitable for utility in power generation parts such as stators, rotors, armatures and actuators or any device whose function is dependent upon an efficient magnetic core, i.e., a magnetic core having a high magnetic moment, minimal magnetic hysteresis and no or little eddy current formation.
2. Discussion of the Background
Many electronic devices rely on magnetic cores as a method of transferring a magnetic field. Due to inefficiency caused by core loss, a portion of this power is lost, typically as waste heat. A core's magnetic properties have the ability to greatly concentrate and enhance magnetic fields. Thus, improving and implementing core materials with low loss as well as high magnetic permeability would enormously enhance the efficiency of the device. With increased interest in environmentally-conscious devices, the implementation of improved magnetic core material across millions and millions of devices that require them (all computers, TVs, cell phones, vehicle power electronics, etc.) could produce significant benefits for global energy conservation.
Magnetic materials generally fall into two classes which are designated as magnetically hard substances which may be permanently magnetized or soft magnetic materials which may be reversed in magnetism at low applied fields. It is important in soft magnetic materials that energy loss, normally referenced as “core loss” is kept to a minimum whereas in hard magnetic materials it is preferred to resist changes in magnetization. High core losses are therefore characteristic of permanent magnetic materials and are undesirable in soft magnetic materials.
Soft magnetic core components are frequently used in electrical/magnetic conversion devices such as motors, generators and transformers and alternators, particularly those found in automobile engines. The most important characteristics of soft magnetic core components are their maximum induction, magnetic permeability, and core loss characteristics. When a magnetic material is exposed to a rapidly varying magnetic field, a resultant energy loss in the core material occurs. These core losses are commonly divided into two principle contributing phenomena: hysteresis and eddy current losses. Hysteresis loss results from the expenditure of energy to overcome the retained magnetic forces within the core component. Eddy current loss, the other source of core loss, refers to circular currents setup within the magnetic core due to the applied magnetic field, as explained by Faraday's Law. Eddy current losses are brought about by the production of induced currents in the core component due to the changing flux caused by alternating current (AC) conditions. These circular currents create a magnetic field anti-parallel to the applied field, decreasing the overall field within the core. In order to reduce eddy current formation, materials with low electrical conductivities are used.
Thus magnetic core inefficiency is measured in terms of core loss. To improve core loss, the magnetic core must demonstrate a reduced measure of magnetic hysteresis as well as lowered eddy current formation. Applicants have described a magnetic core of significantly reduced magnetic hysteresis and low eddy current formation obtained by sintering superparamagnetic core shell nanoparticles having an iron oxide core and silica shell into a monolithic core structure in U.S. application Ser. No. 13/529,316, filed Jun. 21, 2012, the disclosure of which is incorporated herein by reference in its entirety.
These nanoparticles, while offering exceptionally low to zero coercivities (HC), typically have decreased magnetic saturations (MS). One possible reason for this lower magnetic saturation is canted spin alignment due to defects near the surfaces of these nanoparticles. It is believed that defects near the surface (be they crystalline or spin orientation defects) become kinetically trapped during the synthesis of the nanoparticles. Such atomic scale disorder lowers the MS and limits the maximum magnetic flux capacity of a magnetic device such as an inductor.
Thus, the magnetic saturation (Ms) is a second important magnetic property of a material. Magnetic saturation is empirically measured and is representative of the total magnetic moment of a material sample. A low Ms can limit the application utility of a material and therefore, a high Ms is an important property to be an effective and useful magnetic material.
The magnetic saturation is influenced by a number of factors, which includes material composition, crystallinity and the stress-strain exerted on the material during production.
The use of powdered magnetic materials allows the manufacture of magnetic parts having a wide variety of shapes and sizes. Conventionally, however, these materials made from consolidated powdered magnetic materials have been limited to being used in applications involving direct currents. Direct current applications, unlike alternating current applications, do not require that the magnetic particles be insulated from one another in order to reduce eddy currents.
Conventionally, magnetic device parts are constructed from powders by compaction of the powders to a defined shape and then sintering the compact at temperatures of 600° C. or higher. Sintering the part following compaction, is necessary to achieve satisfactory mechanical properties in the part by providing particle to particle bonding and hence strength. However, sintering may cause volume changes and results in a manufacturing process with poor dimensional control.
In other conventional processes designed to prepare parts having minimum eddy current losses, the magnetic particles are coated with thermoplastic materials before pressing. The plastic is provided to act as a barrier between the particles to reduce induced eddy current losses. However, in addition to the relatively high cost of such coatings, the plastic has poor mechanical strength and as a result, parts made using plastic-coated particles have relatively low mechanical strength. Additionally, many of these plastic-coated powders require a high level of binder when pressed. This results in decreased density of the pressed core part and, consequently, a decrease in magnetic permeability and lower induction. Additionally, and significantly, such plastic coatings typically degrade at temperatures of 150-200° C. Accordingly, magnetic parts made in such manner are generally limited to utility in low stress applications for which dimensional control is not critical.
Thus, there remains a need for magnetic powders to produce soft magnetic parts, having increased green strength, high temperature tolerance, and good mechanical properties, which parts have minimal or essentially no core loss and high magnetic moment.
Conventionally, ferromagnetic powders have been employed for the production of soft magnetic core devices. Such powders are generally in a size range measured in microns and are obtained by a mechanical milling diminution of a bulk material. Superparamagnetic nanoparticle materials having particle size of less than 100 nm have found utility for magnetic record imaging, as probes for medical imaging and have been applied for targeted delivery of therapeutic agents. However, the utilization of superparamagnetic powders for production of core magnetic parts has until now, been Limited.
For example, Toyoda et al. (U.S. 2011/0104476) describe a soft magnetic material of iron or an iron alloy particle having a grain size of from 5 to 400 μm which is provided with an oxide insulative coating including silicon oxide. The coated particles are mixed with an organic substance which is a non-thermoplastic resin and at least one of a thermoplastic resin and a higher fatty acid. The content of the organic substance in the mixed material is from 0.001 to 0.2% by mass. The mixed material is compression molded and then subjected to a heat treatment at a temperature between the glass transition temperature and the thermal decomposition temperature of the non-thermoplastic resin. The molded and heat treated structure is indicated to be useful for electric and electronic components such as a motor core or a transformer core.
Hattori et al. (U.S. 2006/0283290) describe silica coated, nitrided iron particles having an average particle diameter of 5 to 25 nm. The particles are “substantially spherical” and are useful for magnetic layers such as a magnetic recording medium.
Ueta et al. (U.S. 2003/0077448) describes a ferromagnetic raw metal powder (primarily iron) having a coating of various oxide materials including silicon. Claim 1 provides a ferromagnetic powder which is surface coated with a silicone resin and a pigment. The coated particle has a diameter on the order of 100 microns. Warm pressing of the powder to produce a core is described as well as annealing of a core at elevated temperature.
Tokuoka et al. (U.S. Pat. No. 7,678,174) describe an iron based powder particle having an iron or iron alloy core and an oxide type insulating coating, including silicon oxide. An ester wax is also added to the particle surface. The coated powder particles are on the order of 200 microns in size as described in Example 1. The lubricated powder is pressure molded to form a molded body and the molded body heat treated.
Blagev (U.S. Pat. No. 5,512,317) describes an acicular magnetic iron oxide particle having a magnetic iron oxide core and a shell containing a silicate compound and cobalt (II) or iron (II) compound as a dopant. The doped acicular particles have a length typically of about 0.15 to 0.50 μm and are employed in magnetic recording media.
Nomura et. al. (U.S. Pat. No. 5,451,245) describes acicular magnetic particles having a largest dimension of about 0.3 μm which are suitable for magnetic recording media. Hydrated iron oxide particles are first coated with an aluminum or zirconium compound, then heated to form a hematite particle. This formed particle is then coated a second time with an aluminum compound followed by a reduction treatment. Silicon compounds may be included in either coating to enhance the properties of the particle.
Soileau et al. (U.S. Pat. No. 4,601,765) describes a core obtained by compaction of iron powder which has been coated with an alkali metal silicate and then a silicone resin polymer. The iron particles to which the coating is applied have a mean particle size of 0.002 to 0.006 inches. The core is prepared by compaction of the powder at greater than 25 tons per square inch and then annealing the pressed component.
Yu et al. (J. Phys. Chem. C 2009, 113, 537-543) describes the preparation of magnetic iron oxide nanoparticles encapsulated in a silica shell. Utility of the particles as magnetic binding agents for proteins is studied.
Tajima et al. (IEEE Transactions on Magnetics, Vol. 41, No. 10, October, 2005) describes a method to produce a powder magnetic core described as warm compaction using die wall lubrication (WC-DWL). According to the method an iron powder coated with a phosphate insulator was compacted under a pressure of 1176 MPa at a temperature of 423° K to produce a core type structure.
Sun et al. (J. Am. Chem. Soc., 2002, 124, 8204-8205) describes a method to produce monodisperse magnetite nanoparticles which can be employed as seeds to grow larger nanoparticles of up to 20 nm in size.
Bumb et al. (Nanotechnology, 19, 2008, 335601) describes synthesis of superparamagnetic iron oxide nanoparticles of 10-40 nm encapsulated in a silica coating layer of approximately 2 nm. Utility in power transformers is referenced, but no description of preparation of core structures is provided.
Mazzochette et al. (U.S. 2012/0106111) describes a magnetic anisotropic conductive adhesive composition which contains an adhesive binder and a conductive nano-material filler. The adhesive binder is a UV, radiation or heat curable resin such as epoxy, acrylate or urethane. The conductive filler particles may be paramagnetic or ferromagnetic and include aluminum, platinum, chromium, manganese, iron and alloys of these. The particles may be coated with a conductive metal such as gold, silver, copper or nickel. In application, the adhesive is applied to the substrate structure, exposed to a magnetic field to align the particles and the resin cured while the field is applied.
Archer et al. (U.S. 2010/0258759) describes metal oxide nanostructures which may be hollow or contain an inner core particle. In one embodiment, this reference describes coating α-Fe2O3 spindle particles with a SiO2 layer, then coating those particles with a SnO2 layer. Porous double-shelled nano-cocoons were prepared by application of two SnO2 layers, annealing the particles at 550 to 600° C. and then dissolving the SiO2 from the particle. The magnetic properties of the particles are mentioned within a general description.
Liu (U.S. 2010/0054981) describes bulk nanocomposite materials containing both hard phase nanoparticle magnetic material and soft phase nanoparticle magnetic material. The two components are mixed and warm compacted to form the bulk material. Prior to the warm compaction, the materials may be heated annealed or ball milled. Liu describes that the density of the compacted bulk material increases with increasing compaction temperature and pressure. The soft phase materials include FeO, Fe2O3, Co Fe, Ni CoFe, NiFe and the hard phase materials include FePt, CoPt, SmCo-based alloys and rare earth-FeB-based alloys. Various methods to prepare magnetic nanoparticles are described, including a “polyol Process.” In Example 1, a bulk nanocomposite of FePt and Fe3O4 is prepared and tested for properties. A phase transition with increasing temperature is confirmed by showing corresponding changes in magnetic properties such as saturation magnetization and coercivity.
Ueta et al. (U.S. 2003/0077448) describe preparation of an iron-based powder having an insulate coating of multiple layers. The iron based powder is first painted with a solvent based silicone resin composition and a pigment. The solvent is dried away and the silicone resin cured. An outer layer of a metal oxide, nitride or carbide is then applied. The coated powder is then formed into a core, optionally with annealing to remove the strain due to pressing. Ueta suggests that the annealing causes thermal degradation of the silicone to form a silica layer including the pigment on the iron base particle.
Bergendahl et al. (U.S. Pat. No. 8,273,407) describe a method to form a thin film of magnetic nanoparticles on a substrate such as a semiconductor wafer. The film contains aggregates of magnetic nanoparticle clusters which are separated from one another by a distance of from 1 to 50 nanometers. Clusters of the magnetic nanoparticles are first applied to the substrate and the clusters are thermally annealed or irradiated with UV or laser to form aggregates. The magnetic nanoparticles may be Fe, Ni, Co, NiCo, FeZn, borides of these, ferrites, rare earth metals or alloy combinations. An insulator coating is placed over the magnetic aggregates. The insulator material may be SiO2, Si3N4, Al2O3, ceramics, polymers, ferrites, epoxies, Teflon or silicones.
Sun et al. (U.S. Pat. No. 6,972,046) describes a process of forming a hard-soft phase, exchange-coupled magnetic nanocomposite. According to the method solvent dispersions of hard phase nanoparticles and soft phase nanoparticles are mixed, and the solvent removed to obtain self-assembled structures. Coatings of the nanoparticles are removed in an annealing treatment to form a compact nanoparticle self-assembly wherein the nanoparticles are exchange coupled. The soft magnetic materials include Co, Fe, Ni, CoFe, NiFe, Fe2O3 and other oxides. The hard magnetic materials include CoPt, FePt, SmCo based alloys and rare earth-FeB-based alloys. The nanocomposites may be compacted to form a high density nanocomposite that is devoid of spaces between the magnetic materials in order to obtain a bulk permanent magnet. Sun et al. describe a direct relationship of coercivity and annealing temperature up to a temperature of agglomeration of the nanoparticles.
None of the above references disclose or suggest that thermal annealing of core shell nanoparticles having an iron oxide core and silica shell results in a significant increase in magnetic saturation. Likewise, none of the above references disclose or suggest a monolithic magnetic core constructed by heated compression of thermally annealed nanoparticular iron oxide encapsulated in a silicon dioxide coating shell, wherein the particles are directly compacted without addition of lubricant or other material to facilitate particle adherence.